The present embodiments relate to a transmit coil arrangement for a magnetic resonance device.
When imaging using magnetic resonance devices utilizing a basic field strength greater than or equal to 3 Tesla, for example, interactions of an object being recorded with excitation fields generated by a high-frequency antenna cause the image quality to deteriorate. This is manifested, for example, in a spatial variation of flip angles during a transmit phase or fluctuations in a signal-to-noise ratio during data acquisition (e.g., during receiving). Absorption of the transmit output by the object to be recorded (e.g., specific absorption rate (SAR)) is also more important here, so some imaging sequences are to be adjusted due to the SAR limitation with the result that the quality of these image recordings is reduced.
To resolve both these problems, the standard transmit coils with circular polarization (e.g., birdcage coils) are no longer used. Instead, transmit coil arrangements configured as antenna arrays are used. In conjunction with a multichannel transmit system, the excitation field and the flip angle distribution may be shaped. At the same time, the SAR load for the object to be recorded (e.g., a patient) may also be reduced. This technique, with which an excitation field is generated at the same time using a plurality of conductor loops forming part of the transmit coil arrangement, may be referred to as parallel transmission. Such arrays have been used in the prior art for receive coil arrangements in order to improve the signal-to-noise ratio and reduce recording times.
A problem with such transmit coil arrangements, which include an array of conductor loops, is the coupling occurring between the individual conductor loops (e.g., antenna elements). Adequate decoupling is provided to avoid feedover between the transmit channels and therefore, in some instances, destruction of the transmit channels. For transmit coil arrangements having conductor loops disposed following one after another over the periphery, measures to achieve adequate decoupling are known. Examples are the use of an overlap between adjacent conductor loops or a capacitor in a shared conductor of adjacent conductor loops.
For transmit arrays, in which antenna elements (e.g., conductor loops) follow one after another in a peripheral direction and a longitudinal direction, the methods known from receive coil arrangements fail, as the preamplifier decoupling used in the known methods may not be used with transmit coil arrangements.
Many trials on the subject of more than two conductor loops of a transmit coil arrangement that follow one after another in a longitudinal direction are based on simulation, with the decoupling of the conductor loops playing no role. In a typical transmit coil arrangement, the conductor loops are decoupled by overlap. However, only adjacent conductor loops may be decoupled, as a very powerful disadvantageous coupling of conductor loops positioned diagonally to one another occurs. Decoupling using a shared capacitor also may not be possible, since with the arrangement of at least three conductor loops following one after another in a longitudinal direction, there is no longer a sufficient number of degrees of freedom for the center ring or rings to allow decoupling.
In one approach to a solution, amplifiers with a low output impedance are used. The amplifiers are amplifiers that operate in the manner of an ideal power source and therefore also emit the required current when individual antenna elements are coupled. A countercurrent, for example, is generated. The countercurrent negates the coupling effects. The preamplifier, for example, is to be configured so that the preamplifier manages with the output coupled over, and large dimensions may therefore be required. Such compensation for inadequate decoupling of the conductor loops by increasing amplifier output is disadvantageous, as the outlay for this purpose and the costs required rise excessively so that the use of such a transmit coil arrangement is not practical. An embodiment with amplifiers of low output impedance is described, for example, in the Proceedings of the International Society of Magnetic Resonance in Medicine 2007, Abstract 172 (see also the article “Ultra-low Output Impedance RF Power Amplifier Array” by X. Chu, et al., Proc. Intl. Soc. Mag. Reson. Med. 15 (2007), page 172).